First you need to phone your AFCO and tell them that you want back into the army. They will then get you in for an interview and ask you why you want back in etc what has changed this time. If the recruiter is convinced he will then send of a 203 to Glasgow where they will determine if you get back in they look at your reports and records from training. Beware though to get a reply from Glasgow can take a long time and sometimes its not always the answer you want it to be.
It is possible, I DAOR'd in 2009 from Harrogate (I was a bit of a fanny at sixteen) and i'm re-enlisting and starting the ITC in July. It's not a sure thing and even finding out if they want you back can be a long process, especially if you had any injuries during training like I did. Get to your AFCO or give them a ring as UAO has said and have a chat. Best of luck.